1. FIELD OF THE INVENTION
The present invention relates to a ceramic semiconductor device exhibiting a positive temperature coefficient of resistance (hereinafter referred to as positive ceramic semiconductor device) which can be used as heat generating elements of various types or as current control elements in electric circuits.
2. DESCRIPTION OF THE RELATED ART
The hitherto known positive ceramic semiconductor device is typically of such a structure which has a pair of electrodes each of a two-layer structure composed of a nickel layer and a silver layer implemented by forming first the nickel layer on each of upper and lower surfaces of a positive ceramic semiconductor substrate, and then forming the silver layer over the surface of the nickel layer (reference may be made to Nos. JP-B-58-7044 and JP-A-47-2713).
In the hitherto known positive ceramic semiconductor device of the structure mentioned above, there takes place so-called silver-migration phenomenon in which silver constituents in the silver layer migrate along the surface of the substrate from the electrode serving as positive pole toward the electrode serving as negative pole when a predetermined potential difference is applied across the paired electrodes of positive and negative poles, respectively. The migration of silver is significantly accelerated in the atmosphere of high temperature and high humidity or moisture. This phenomenon is often accompanied with formation of short-circuit between the electrodes, degrading thus performance of the positive ceramic semiconductor device.
Further, it is observed in the positive ceramic semiconductor device that when a current flows through the semiconductor substrate, the current flow is locally concentrated, giving rise to a localized heat generation. As the result, a crack is produced in the ceramic semiconductor substrate due to thermal stress, possibly incurring unwanted degradation in the mechanical strength of the substrate.
Under the circumstances, there exists a demand for improving the positive ceramic semiconductor device so as to exhibit stable characteristics by suppressing as perfectly as possible the silver-migration phenomenon and at the same time preventing the thermal destruction of the semiconductor substrate due to the localized heat generation.
The present invention has been made with a view to satisfying the demand mentioned above.